Monoclonal antibodies are routinely used in vitro for diagnosis and in vivo for diagnosis and immunotherapy. Monoclonal antibodies are used to screen for cancer biomarkers in high risk patients, HIV or infectious hepatitis in our nation's blood supply and many other applications. Knowledge of the epitopes actually being screened for in these diagnostic kits is vital because sequence variations in the antigens being measured could lead to false negative results. This is particularly relevant with the AIDS virus because the virus mutates constantly to avoid recognition by the host immune system. Thus, the optimal development of in vitro diagnostic kits which utilize monoclonal antibodies to assay blood for the presence of HIV necessitates that the epitope being recognized by the antibody is known and is located in a region of the antigen which undergoes no sequence variation. If the epitope which is being recognized by the monoclonal antibody comes from a variable region, mutations may occur in the protein sequence of the HIV protein and be missed by the diagnostic kit, leading to tainted blood supplies. Knowledge of the epitopes enables the development of totally synthetic vaccines without the need for the inactive pathogen. Synthetic immunogens have been shown to induce virus neutralizing antibodies. The advantage of not using a deactivated pathogen for the development of a vaccine eliminates the possibility of active pathogen contaminating the vaccine. If the wrong epitope is chosen for preparation of a synthetic HIV vaccine, deleterious affects can manifest themselves. Not all antibodies generated by the host immune system are neutralizing. Some antibodies have been suggested to enhance viral uptake by the Fc receptor, enhancing infection with HIV. Finally, the use of antibodies as magic bullets have not yet seen widespread use. Before monoclonal antibodies can be utilized as pharmacological agents in humans, their characteristics must be well documented and, of course, this includes a thorough knowledge of the epitope they recognize. We have identified the core epitope on recombinant HIVIIIB p26 identified by the monoclonal antibody 13-102-100 as being residues 102-112. We are currently probing the epitopes on HIVIIIB p26 recognized by other monoclonal antibodies and also epitopes of the envelope glycoprotein gp120.